A long time ago I did some proof of concept work to do GPGPU operations in the browser with regular webGL, it kind of worked: http://learningwebgl.com/blog/?p=1828#comments

I did look into using the same technique to implement bitcoin mining in glsl. My opinion is that it is possible, but not straight forward and probably not worth it, but still a pretty f'ing cool concept.

All you need are shaders to do gpu computation.

People did GPGPU before the likes of OpenCL and CUDA.

well yes... just not from a browser.